This invention relates to a signal amplifying circuit for amplifying an AC signal obtained from a unipolar signal having a DC signal overlapped with an AC component, and more particularly to an improved signal amplifying circuit for for amplifying said AC signal and generating a highly accurate output waveform.
In general, a bar-code reading device is constructed such that the desired character information is provided by codes having, in combination, a plurality of bars (having a high rate of optical absorption) of various widths and spacings (having a high rate of optical reflection) of various widths, wherein a light is irradiated against a scanned bar-code surface optically recorded with the character information, and a variation of reflected light from the bar-code surface is detected and converted into an electrical signal through an optical sensor (a photoelectrical conversion element) so as to read the bar-code.
An electrical signal generated by an optical sensor through a scanning operation over the bar-code surface comprises a unipolar AC component of small amplitude, wherein said AC component is intermittently varied in response to the reflected light from the bar codes, and a high DC component caused by external light, "dark" current through the optical sensor, and the like. Therefore, in order to read the bar-codes and interpret the codes, it is necessary to amplify the above-mentioned AC component up to a desired level and to make the amplified AC signal capable of being processed by a binary circuit for detecting the various bar widths in the code.
A signal processing circuit for processing a signal generated from an optical sensor in the conventional type of bar-code reading device described above will be described with reference to FIGS. 4 to 6.
FIG. 4 is a schematic diagram of a signal processing circuit having a signal amplifier of a conventional DC junction type (hereinafter called "DC type"), wherein a light irradiated against a bar code surface (not shown) by light emitting diode (LED) 1, emitting light in response to an electric current supplied from positive power supply terminal +B through an electric current limiting resistor, is reflected in response to a reflection factor of the bar code surface. This reflected light is scanned in sequence and made incident to phototransistor 2 acting as a light receiving element. This NPN type phototransistor 2 has a collector connected to the power supply terminal +B through a resistor. An emitter voltage of phototransistor 2 at point "a" is varied in response to the amount of reflected light, where an increased collector current results from an increase in the amount of reflected light. In case of sensing a black bar, the voltage at point "a" would thus decrease The emitter voltage (assuming no other element is connected to point "a") consists of a DC component, resulting from external light and a "dark" current through phototransistor 2, overlapped with an AC component corresponding to the variation in the amount of light reflected from the scanned bar-code surface. This DC component is substantially varied in response to a variation in the strength of any external light. A predetermined fixed bias voltage is added to the emitter voltage of phototransistor 2 by means of a bias circuit 3 so that the operating voltage to be applied to an inverting amplifier circuit 4 may be maintained substantially constant. Thus, the inverting amplifier circuit 4 amplifies a signal relative to a reference voltage provided by the bias voltage and hence amplifies only the AC component of the signal.
In the signal amplifier circuit of the DC coupling type shown in FIG. 4, a signal relative to an operating voltage set by the bias circuit 3, as shown by waveform (a) of FIG. 5, is applied to inverting amplifier circuit 4. Accordingly, if inverting amplifier circuit 4 accomplishes an amplifying operation relative to the operating voltage, only an AC component will be inverted and amplified by the inverting amplifier circuit 4. However, if the DC component changes due to a change of disturbant light or the like, the operating voltage changes accordingly. As a result, not only the AC component of the voltage at point "a" will be amplified but also the DC component due to a difference between the operating voltage and the bias voltage will be amplified. If the DC component is large, it is a potential problem that the amplified signal is distorted at the maximum amplitude of the output of inverting amplifier circuit 4, as shown by waveform (b) of FIG. 5, and hence the AC component cannot be amplified with accuracy. It is to be noted that FIG. 5 and FIGS. 2 and 7, which will be hereinafter described show waveforms of signals where bar-codes are represented by black marks on a ground of white.
A signal amplifier circuit of the AC coupling type, shown in FIG. 6, will now be described. The signal amplifier circuit of FIG. 6 includes a phototransistor 2, the emitter voltage of which is applied to an inverting amplifier circuit 4 via a capacitor 5. Thus, a DC component of the signal is removed by capacitor 5 and hence only an AC component is amplified by inverting amplifier circuit 4. The signal amplifier circuit of FIG. 6 further includes a binary digitizing circuit 6 for converting the output of the inverting amplifier circuit 4 into a binary signal, and an automatic threshold level setting circuit 7 for setting a threshold level of the binary digitizing circuit 6 in response to an output of the inverting amplifier circuit 4.
Further, in the signal amplifier circuit of the AC coupling type shown in FIG. 6, a signal developed from phototransistor 2 presents a waveform as shown by (a) of FIG. 7 wherein the amplitude appears on only the lower side of the operating voltage. Accordingly, as the DC component is removed by the capacitor 5, the signal which has passed the capacitor 5 presents a waveform as shown by (c) of FIG. 7 wherein the operating voltage gradually rises in accordance with an attenuation characteristic. Thus, if the signal which has passed the capacitor 5 is amplified by the inverting amplifier circuit 4, the latter will provide an output having an attenuation characteristic as seen from a waveform (d) of FIG. 7. Then, if the output of the inverting amplifier circuit 4 is binary digitized by the binary digitizing circuit 6 with a predetermined fixed threshold level, the output of the binary digitizing circuit 6 will present variation in width in accordance with the attenuation characteristic, resulting in failure to attain accurate binary digitization corresponding to the width of the AC component appearing in the emitter voltage of phototransistor 2. Accordingly, the automatic threshold level setting circuit 7 is necessitated to set a threshold level which follows in accordance with the attenuation characteristic. However, it is a problem that the automatic threshold level setting circuit 7 is complicated in construction.